In the first process step, a parison is formed in a blank mold by vertically advancing a plunger into the blank mold to fill the volume between the blank mold and the plunger with the molten glass. When the parison is formed (the volume is filled) further advancement of the plunger will tend to open the mold and this is undesirable. For a pneumatic plunger mechanism, the plunger-up-pressure has to be adjusted manually from time to time in order to ensure proper parison forming, due to changes in glass viscosity and friction in the plunger mechanism, positioner and guide ring. With an electronic plunger mechanism, the force controller of the servo-electric-plunger mechanism will operate with a selected force demand value or curve until manually adjusted by the operator.
In a conventional plunger mechanism, vertical displacement begins at a “down” position. The first segment of the displacement of the plunger is vertically upwardly from the down position to a “load” position where it will have slightly entered the blank mold. When a molten gob is then dropped into the mold, some of the molten glass will flow downwardly past the top of the plunger thereby beginning the process of forming the finish (threaded portion) of the bottle. The second segment of displacement is rapid vertical displacement to a location where “finish press” is to begin. At this finish press position, a press force will be applied until the parison has been formed whereupon the plunger has reached the “parison formation point” whereat all air has been removed from the mold. Following this, as pressure continues to be applied, the plunger rises slowly as the blow parison cools and shrinks. At some time following the parison formation point, the plunger reaches the up position. It will then be displaced vertically downwardly to the “down” position thereby allowing the parison to be transferred to the blow station where it will be blown into a bottle.